Galaxy may have 11 billion Earth-like planets

David Perlman

Updated 7:37 am, Tuesday, November 5, 2013

This artist's rendition shows Kepler-69c, an Earth-like planet in the habitable zone of a star like our sun, located about 2,700 light-years from Earth.
Photo: Associated Press

This artist's rendition shows Kepler-69c, an Earth-like planet in...

This is a NASA illustration of the newly discovered 6-planet "solar
system" around a sun-like star 2000 light years away.
Photo: Nasa, NASA

This is a NASA illustration of the newly discovered 6-planet "solar...

This artist's rendering provided by NASA shows the Kepler space telescope. NASA is calling off all attempts to fix the crippled space telescope. But it's not quite ready to call it quits on the robotic planet hunter. Officials said Thursday, Aug. 15, 2013 they're looking at what science might be salvaged by using the broken spacecraft as is.
Photo: Uncredited, Associated Press

This artist's rendering provided by NASA shows the Kepler space...

This illustration provided by the Harvard-Smithsonian Center for Astrophysics shows artist's renderings of planets Kepler-20e and Kepler-20f compared with Venus and the Earth. Scientists have found the two Earth-sized planets orbiting a distant star, an encouraging sign for prospects of finding life elsewhere. The discovery shows that such planets exist and that they can be detected by the Kepler spacecraft, said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. They're the smallest planets found so far outside the solar system. Scientists are seeking Earth-sized planets as potential homes for extraterrestrial life, said Fressin, who reports the new findings in a paper published online Tuesday, Dec. 30, 2011 by the journal Nature.
Photo: Associated Press

This illustration provided by the Harvard-Smithsonian Center for...

Comfortably Circling within the Habitable Zone - This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth.
Kepler-22's star is a bit smaller than our sun, so its habitable zone is slightly closer in. The diagram shows an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star. It's about 2.4 times the size of Earth.
Photo: NASA/Ames/JPL-Caltech

Comfortably Circling within the Habitable Zone - This diagram...

This handout illustration provided by San Diego State University, shows a newly discovered planet, called Kepler 35, that circles not one but two stars. Scientists thought this type of two-sun system _ made famous as the home planet of the fictional Luke Skywalker _ is too unstable to support planets. But so far they've found three of these planets with two suns, showing that planets seem to be everywhere. The study is in this week's journal Nature.
Photo: Lynette Cook, Associated Press

This handout illustration provided by San Diego State University,...

This artist's concept illustrates Kepler-16b, the first planet known to definitively orbit two stars -- what's called a circumbinary planet. The planet, which can be seen in the foreground, was discovered by NASA's Kepler mission.
The two orbiting stars regularly eclipse each other, as seen from our point of view on Earth. The planet also eclipses, or transits, each star, and Kepler data from these planetary transits allowed the size, density and mass of the planet to be extremely well determined. The fact that the orbits of the stars and the planet align within a degree of each other indicate that the planet formed within the same circumbinary disk that the stars formed within, rather than being captured later by the two stars.
Photo: NASA/JPL-Caltech/T. Pyle, NASA

Our Milky Way galaxy is crowded with far more habitable Earth-like planets than previously thought - at least 11 billion of them in orbit around distant stars, a team of planet hunters led by UC Berkeley astronomers said Monday.

Erik Petigura, a Berkeley graduate student, analyzed data from the Kepler spacecraft and calculated that at least 50 billion stars much like Earth's sun are blazing throughout the galaxy. Kepler itself has been crippled since last summer by damage to its steering gear and is no longer providing new information to earthbound scientists.

But based on current data provided by Kepler and its telescope over the past four years, Petigura estimates that 11 billion planets roughly the size of Earth are flying in orbits around those suns - at distances that make temperatures on the planet neither too hot nor too cold for liquid water to exist.

Astronomers call that kind of orbit the "Goldilocks zone," and it's where astronomers will focus their search of life, they say.

It's possible that even more Earth-like planets exist than Petigura estimates, for his analysis takes into account only one class of hot stars that are known to be very much like our sun. There are other stars called red dwarfs that are about the size of our sun only cooler, and many so-called "exoplanets" may be circling them too, he said.

When those red dwarf stars are included, there may be as many as 40 billion Earth-size planets in habitable zones of the Milky Way with mild temperatures that are similar to climates on Earth, Petigura said.

Petigura's colleagues are Andrew Howard, a former Berkeley postdoctoral fellow now at the University of Hawaii, and Geoffrey Marcy, the Berkeley professor and pioneer planet hunter who has been leading the search for "exoplanets" since the first was discovered 18 years ago.

Their report is published this week in the Proceedings of the National Academy of Sciences, and they discussed it Monday during a news conference.

Marcy called the Kepler spacecraft, with its remarkable telescope, "the best planet-hunting machine ever."

Its flood of data, he said, is answering the question countless others around the world are asking: "whether our planet Earth is some kind of cosmic freak, or instead is a common occurrence within our Milky Way galaxy."

The astronomers are joining 400 other scientists reporting new results this week at the second international Kepler Science Conference at NASA's Ames Research Center in Mountain View.